Generating sets (gensets) are used to provide electricity for distributed power generation systems which include prime power, standby generation, and network support. De-regulation of electric utilities has resulted in many customers utilizing their standby diesel gensets to improve power quality or avoid peak electricity tariffs.
A genset often consists of a diesel engine, a synchronous machine, and two controllers: a speed governor and an automatic voltage regulator. The synchronous machine employs a salient-pole rotor. FIG. 1 illustrates a prior art system 20 genset arrangement. Basic components of system 20 include components also indicated in FIG. 2 by the same numbers. Those components include a generator 28, an exciter 26, an automatic voltage regulator, (AVR) 22, an amplifier 24, a speed governor 30 and its related fuel pump 32. The governor 30 has been used to maintain constant generator speed ω 52. Governor 30 responds to changes in generator speed ω 52 to act as a feedback controller to control the fuel rate of the fuel pump 32 to thereby minimize deviations by a sudden change in the genset's real-power load.
In FIG. 1, Vref 34 is a generator voltage reference, Vt 36 is the generator terminal voltage and Efd is exciter 26 field voltage 37. Pe 40 represents the real power load, determined from Vt2/R, where R is a resistive load. Further, in ½H 42, H represents the total moment of inertia of the diesel engine and generator rotating parts, Tm 44 represents mechanical torque as to the diesel engine and generator rotating parts, Tf 46 represents friction torque the diesel engine and generator rotating parts, Tmax 48 represents maximum torque as to the diesel engine and generator rotating, and ωref 50 is the generator speed reference, and ω 52 represents the generator speed. The “s” in the 1/s block 54 is the Laplace operator (also sometimes shown throughout the Figures as “S”).
The AVR 22 maintains constant generator terminal voltage by controlling the field current to the exciter 26 through feedback control by summing 56 the generator terminal voltage Vt 36 with the generator voltage reference Vref 34. The generator terminal voltage Vt 36 is determined by multiplying 58 the generator output voltage with the generator speed ω 52. Some modern microprocessor-based AVRs are implemented with Proportional, Integral, and Derivative (PID) control for stabilization and various supplemental control systems. Such known digital regulators have used a PID controller 23 in the forward path as shown in the FIG. 1 prior art system 20 illustration. Such a PID control can be implemented within the AVR. Such prior art gensets such as shown in FIG. 1 include limiters, a var/power factor controller, tuning functionality, protection, and monitoring features, as disclosed in the article: K. Kim, M. J. Basler and A. Godhwani, “Supplemental Control in a Modern Digital Excitation System”, IEEE Power Engineering Society Winter Meeting 2000, Singapore.
The real power load Pe 40 is fed to the speed control loop through the 1/ω 52, shown as block 60. The nominal value of the generator speed ω 52 is 1.0 per unit. The 1/ω block 60 clarifies the unit conversion from electric power to torque for the speed control loop. The speed control loop provides feedback control of the generator speed ω 52 by subtracting 62 the generator speed ω 52 from the generator speed reference ωref 50.
Unlike large generators, many gensets are expected to change operation from no load to full load in a single step-load application. This can cause large changes in generator speed ω 52 or stalling of the engine.
A sudden increase in the genset's real-power load causes an increased load torque on the engine. Since the load torque exceeds the engine's torque and the engine governor cannot respond instantaneously, the generator speed ω 52 decreases. After detecting this deceleration, the governor increases the fuel supplied to the engine. Since the generated voltage is proportional to generator speed ω 52, the generator output voltage decreases due to armature reaction and internal voltage drops. The voltage regulator compensates by increasing the machine's field current. FIG. 1 shows simplified genset models with cross coupling when resistive load is applied, through the interaction between voltage and speed control.
International Standard ISO8528-5, “Reciprocating Internal Combustion Engine Driven Alternating Current Generating Sets—Part 5: Specification for Generating Sets”, 1993, is used to assess diesel genset performance. A genset is classified based on a series of key performance indicators. For modern gensets with a G2 classification, the maximum voltage deviation from the nominal setpoint for a sudden load acceptance shall not exceed 20%. The maximum electrical frequency deviation shall not exceed 10%. Voltage recovery time must be less than six seconds and frequency recovery time must be less than five seconds. Since real power is proportional to the square of the voltage, a fast acting AVR significantly impedes generator speed ω 52 recovery by quickly recovering the voltage; hence, placing more load on the engine.
A common way to reduce generator speed ω 52 drop is to provide an additional voltage dip during speed drops. This allows faster engine recovery by reducing real power. Various voltage setpoint adjustments (under-frequency schemes) are used in modern AVRs. A Load Adjustment Module (LAM) is also suggested in the article: K. D. Chambers, D. J. McGowan, and D. J. Morrow, “A Digital Load Relief Scheme for a Diesel Generating Set”. IEEE Transactions on Energy Conversion, Vol. 13, No. 2, June 1998, incorporated herein fully by reference, which temporarily reduces voltage during a transient, and therefore aids generator speed ω 52 recovery. However, the governor's reaction to a change in generator speed ω 52 is much slower than a change to real power. Furthermore, the voltage loop is affected by the speed response because generator voltage is proportional to generator speed.
As noted above, in genset control systems the AVR 22 provides voltage regulation and the governor 30, controls the speed of the engine when the genset is operating in an island mode. However, a conventional design practice is to design the AVR and governor independently even if there is an interaction between voltage and speed control as shown in FIG. 1. Thus, it is common to have situations where this creates problems such as during a factory-load acceptance test for a manufactured genset.
Engine performance is nonlinearly affected by changes in operating speed and load. A supervisory control to consider the smoke and torque limit map was introduced, such as discussed in the publication A. R. Cooper, D. J. Morrow and K. D. R. Chambers, “Development of a Diesel Generating Set Model for Large Voltage and Frequency Transients”, IEEE Transactions on Energy Conversion, Vol. 13, No. 2, June 1998. Generator speed ω 52 response to large real load is nonlinear, and it is also affected by voltage regulator response, as discussed in the said Cooper publication, and in Seung-Hwan Lee, Jung-Sik Yim, Joon-Hwan Lee and Seung-Ki Sul, “Design of Speed Control Loop of A Variable Speed Diesel Engine Generator by Electric Governor,” Industry Applications Society Annual Meeting, 2008. IAS '08. IEEE, pp. 1-5, 2008.
When a large real-power load is applied, the generator speed is reduced, and consequently so is the generator voltage. Thus, the governor increases engine fueling to maximum, and the AVR increases the excitation to maintain the terminal voltage at rated value. The reaction of the AVR deteriorates the speed dip and recovery.
A conventional control system and method implements various under-frequency characteristics to improve speed performance by coordinating a trade-off between generator speed ω 52 and voltage deviation. Unfortunately, it is not easy to obtain the parameters for this approach to achieve the required performance. The AVR is designed based on linear fashion, even if the genset control systems become nonlinear as a result of interaction of the generator voltage and the generator speed ω 52 loop. Thus, using control outputs based on the linear fashion can cause overshoots in both the generator speed and voltage regulation loops.